The above paper by Schwartz et al. recently demonstrates time stretching of RF signals entirely in the electronic domain 1, which is in contrast to the large body of work that implements RF time stretching in the optical domain. Such strict RF technology allows for simplified integration accompanied by a reduction in system complexity. Unfortunately, the above paper 1 mixes discussion of the "photonic time-stretch" technique 2-5 with discussion of RF 6, 7 and optical 8-10 implementations of the "time-lens" technique. This leads to incorrect conclusions about the limitations of the photonic time-stretch technique. We intend to discuss subtleties of these technologies that will then clarify the limits and applicability of each technique. The time-stretching system realized in the above paepr by Schwartz et al. 1 is shown schematically in Fig. 1, which is functionally identical to the first figure of Caputi's seminal work 7 (although identical variables take different meanings in each study). In these systems, a time-limited input signal first propagates through a medium with dispersion d_(1) (seconds/hertz). The "time lens" then applies a quadratic phase modulation in time to the dispersed signal 11. In the RF implementation, this phase modulation is accomplished by mixing a chirped reference signal with the input signal, where the chirped reference is generated by an impulse that traverses a medium of dispersion d_(2). In the optical implementation, the technique is the same, although the phase is typically directly modulated using a conventional phase modulator with a quadratic drive signal. The signal is then "focused" (or "compressed") to the stretched output by a medium with dispersion d_(3) velence -(d_(1)~(-1) - d_(2)~(-1))~(-1) and square-law detected. This effectively stretches features in the input of length (DELTA)t_(1) to length (DELTA)t_(2) velence d_(2)/(d_(2) - d_(1))(DELTA)t_(1).
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